Hey Kurgans,
Go to Virtuemart in the back-end.
Tab "Administration" -> "Configuration" in here go to Tab "Site".
At the right bottom you can set the dimensions. Also check the option above the dimensions (auto resize).
Now connect with you FTP server and go to:
/components/com_virtuemart/shop_image/product
In there you find the folder "resized" rename it to "resized-" or something else.
Now, as you browse, Virtuemart will generate a new "resized" folder and thumbnails.
Cheers,
Robert